Semiconductor device, method of forming epitaxial film, and laser ablation device

ABSTRACT

The present invention provides a semiconductor device comprising a single-crystal silicon substrate; and a single-crystal oxide thin film having a perovskite structure formed through epitaxial growth on the single-crystal silicon. substrate. The single-crystal oxide thin film is directly in contact with a surface of the single-crystal silicon substrate, and contains a bivalent metal that is reactive to silicon.

BACKGROUND OF THE INVENTION

[0001] The present invention generally relates to semiconductor devices,and, more particularly, to a semiconductor device that has an oxide filmformed by a silicon substrate and an epitaxial film grown on the siliconsubstrate.

[0002] Conventionally, the formation of an oxide film on a siliconsubstrate has been commonly performed. For most cases, the oxide film isan amorphous film, and is mainly employed as an insulating film or adielectric film.

[0003] In a semiconductor device that utilizes the properties of anoxide film, such as a ferroelectric memory, a crystallized oxide film isemployed to realize desired functions. Some oxide crystals exhibit manyproperties including not only insulation and dielectric properties butalso ferroelectricity, piezoelectricity, pyroelectricity, andsuperconductivity. By forming oxide crystals having these properties asa thin film on a single-crystal silicon substrate, a device havingvarious functions such as a memory, a sensor, and a filter, can beobtained. These functions derive from the properties of the oxidecrystals. In an amorphous state, however, the oxide films cannot exhibitthe properties or can exhibit only a part of the properties.

[0004] A ferroelectric film used in a ferroelectric memory obtains theabove properties through crystallization in the existence of oxygen at atemperature of several hundred degrees centigrade. However, aconventional ferroelectric film is a polycrystalline film, in which theorientations of crystals in a direction perpendicular to the substrate,for instance, are aligned, but the orientations of crystals in otherdirections are generally at random, resulting in defects with the grainboundaries, for instance. For this reason, a semiconductor deviceincluding a conventional crystalline oxide film only has a limitedability to exhibit the properties of the oxide crystal.

[0005] Meanwhile, it has been very difficult to form an oxide filmhaving an epitaxial orientation in which the crystal orientations arealigned not only in a direction perpendicular to the substrate surfacebut also in a direction parallel to the substrate surface.

[0006] To develop an epitaxial oxide thin film on a single-crystalsilicon substrate, it is necessary to utilize the orientations on thesurface of the single-crystal silicon substrate. However, asingle-crystal silicon substrate has the same chemical properties asmetals. If exposed to an oxygen atmosphere at a high temperature, thesurface of a single-crystal silicon substrate is quickly oxidized toform a silicon oxide (SiOx) film. Since a silicon oxide film isamorphous and does not have a crystal orientation, an epitaxial oxidethin film cannot grow on a silicon oxide film. It is also essential forepitaxial growth that the reaction and diffusion between a growing thinfilm and a single-crystal silicon substrate should be minimized. Forthis reason, not all oxides can be formed through epitaxial growth on asingle-crystal silicon substrate. Materials that are known to date assuitable for epitaxial growth on a single-crystal silicon substrate onlyinclude rare earth element oxides such as yttrium fully stabilizedzirconia (YSZ: see J.Appl.Phys. vol.67, (1989) pp.2447), magnesia spinel(MgAl₂O₄: ISSCC Digest of Tech. Papers (1981) pp.210), and cerium oxide(CeO₂: Appl.Phys.Lett, vol.56, (1990) pp.1332), and strontium titanate(StTiO3: Jpn.J.Appl.Phys, 30 (1990) L1415).

[0007] The index of crystal quality of an epitaxial oxide thin filmformed on a silicon substrate is a half value width that is obtainedthrough X-ray diffraction (Full Width at Half Maximum, FWHM). A halfvalue width is a value determined from a rocking curve obtained byscanning fixed 2 θ axes of an X-ray peak. More specifically, the halfvalue width (FWHM) is determined by the peak width at a half of thestrength of the peak top of the rocking curve. This indicates the degreeof crystal tilt in the thin film. A smaller value indicates propertiessimilar to those of a single-crystal material, which exhibits bettercrystallization and orientation. With aligned orientations of crystalsin a thin film, the electric properties (such as leak properties withimproved hysteresis properties) are improved. It is therefore essentialthat a thin film having as small half value width (FWHM) as possibleshould be produced for suitable application to a device.

[0008] Materials having perovskite structures, including bariumtitanate, are ferroelectric materials that are desirable in terms ofpiezoelectricity, dielectricity, pyroelectricity, semiconductivity, andelectric conductivity. However, it has been conventionally difficult todevelop a material having a perovskite structure through epitaxialgrowth directly on a single-crystal silicon substrate. This is becausean amorphous-phase SiOx film or a reaction phase such as silicide isformed on a single-crystal silicon substrate.

[0009] The only epitaxial perovskite thin film conventionally employedand formed on a single-crystal silicon substrate is strontium titanium(SrTiO₃). A metal strontium film as an intermediate layer is interposedbetween a strontium titanium thin film and a single-crystal siliconsubstrate. Since titanium and silicon are reactive to each other, astrontium titanate film is formed to prevent reaction between titaniumand silicon. More specifically, after a metal strontium film is formedon the surface of a silicon substrate, strontium and titanium aresupplied in the existence of oxygen so as to produce a strontiumtitanate film. If the metal strontium film as an intermediate layer isthin enough, the titanium diffuses into the metal strontium film duringthe formation of a SrTiO₃ film. As a result, a SrTiO₃ film that appeasesto have developed through epitaxial growth directly on thesingle-crystal silicon substrate can be obtained.

[0010] To develop a strontium titanate film through epitaxial growth inthe above manner, a process control is necessary at the atomic layerlevel, and, therefore, a molecular beam epitaxy (MBE) technique isemployed. Alternatively, Japanese Laid-Open Patent Application No.10-107216 discloses a method of forming a strontium titanate (SrTiO₃)film. More specifically, high-vacuum laser ablation is performed on aSrO target in a high vacuum of 10⁻⁸ Torr, so as to form a strontiumoxide (SrO) film as an intermediate layer. A strontium titanate (SrTiO₃)film is then formed on the SrO film. If the SrO intermediate layer isthin enough, the titanium diffuses into the SrO intermediate layerduring the formation of the SrTiO₃ film. As a result, it appears thatthe SrTiO₃ film has developed through epitaxial growth directly on thesingle-crystal silicon substrate.

[0011] Alternatively, another method has been suggested in which anintermediate layer is formed to prevent the reaction between asingle-crystal silicon substrate and a perovskite oxide, and theformation of a SiOx phase. Intermediate layers that are known to dateinclude yttria partially stabilized zirconia (YSZ: J.Appl.Phys.67 (1989)pp.2447) and magnesia spinel (MgAl₂O₄: ISSCC Digest of Tech. Papers(1981) pp.210). With any of these intermediate layer, a 2-layeredstructure in which the intermediate layer and a perovskite film arelaminated in this order on a single-crystal silicon substrate isobtained.

[0012] A yttria partially stabilized zirconia (YSZ) thin film formedthrough epitaxial growth on a single-crystal silicon substrate isobtained by a pulse laser deposition technique using YSZ ceramics as atarget. Where a perovskite film is formed on such a YSZ film on asingle-crystal silicon substrate, an epitaxial phenomenon in which the(011)-plane of the perovskite film is orientated in a directioncorresponding to the (001)-plane of the YSZ film can be seen. However,the spontaneous polarization direction of a tetragonal perovskite filmis the <001>-direction. If the (011)-plane of a perovskite film isorientated, the spontaneous polarization direction is tilted by 45degrees with respect to the substrate surface. In such a case, theapparent polarization in the direction perpendicular to the substratesurface decreases, which is disadvantageous in application to a FeRAM ora piezoelectric actuator.

[0013] Conventionally, it has been known that an oxide thin filmcontaining a rare earth element such as cerium oxide (CeO₂) or yttriumoxide (Y₂O₃) can be formed through epitaxial growth on a single-crystalsilicon substrate by a pulse laser deposition technique using acomposite material of the rare earth element as a target. However, suchan oxide thin film containing a rare earth element is (011)-orientatedwith respect to a single-crystal silicon substrate. For this reason, itis difficult to form a (001)-orientated perovskite film throughepitaxial growth on such an oxide thin film.

[0014] There is also a known method of forming a MgAl₂O₄ film throughepitaxial growth on a single-crystal silicon substrate by a CVDtechnique. As disclosed in J.Appl.Phys. vol.66 (1989) pp.5826, aperovskite film formed on a MgAl₂O₄ film in this manner has the<001>-direction aligned with respect to the <001>-direction of MgAl₂O₄film. This is advantageous in application to a FeRAM or a piezoelectricactuator.

[0015] As described above, the only known perovskite oxide film that canbe formed through epitaxial growth directly on a single-crystal siliconsubstrate is a strontium titanate film formed with a thin intermediatelayer. Also, the methods of producing such a film only includes a MBEtechnique that requires a high vacuum of 10⁻¹² Torr or lower, and apulse laser deposition technique that also requires a high vacuum. Ahigh-vacuum pulse laser deposition technique requires a vacuum of 10-8Torr or lower, and, therefore, a metal-sealed vacuum device is necessaryto perform high-vacuum pulse laser deposition. Furthermore, such ahigh-vacuum process requires maintenance such as baking, and lowers thethroughput, resulting in an increase of production costs.

[0016] In the MBE technique, metal strontium is used as a raw material.However, an alkaline earth metal such as metal strontium quickly reactsto water, and therefore needs to be stored in oil. With metallicmagnesium, there is also a problem in safety, because metallic magnesiumeasily starts fire. Meanwhile, strontium oxide is used as a target usedin a high-vacuum pulse layer deposition technique. However, an oxidecontaining an alkaline earth metal such as strontium oxide hasdeliquescence, and absorbs moisture from the air to change into ahydroxide. Such a hydroxide containing an alkaline earth metal is astrong alkali and therefore corrodes the device. As described so far,with the conventional methods and techniques, there are problems insafety and maintenance, as intensive care must be taken in handling rawmaterials.

[0017] In the MBE technique, it is difficult to form a thickintermediate layer such as a perovskite film or a strontium oxide film,because each layer is atomically laminated on one another. In the CVDtechnique, on the other hand, an organic metal material is supplied inthe existence of oxygen, and is decomposed on the substrate surface soas to form a deposition film on the silicon substrate. A cyclepentadienecompound is often employed as the organic metal material. Since such amaterial does not contain oxygen atoms, however, it is difficult tocombine with oxygen at the time of decomposition, often resulting inprecipitation of the metal. While an oxide does not easily react tometal, two metallic materials easily diffuse into the substrate to forman alloy. Such a reaction layer disturbs the surface crystal, andtherefore hinders the epitaxial growth of a single-crystal oxide thinfilm on the surface. Further, it is difficult to maintain a high-volumechamber at a high vacuum in accordance with the MBE technique and thehigh-vacuum pulse laser deposition technique. As a result, it is alsodifficult to form an oxide film through epitaxial growth on asingle-crystal silicon substrate having a large diameter.

[0018] Japanese Laid-Open Patent Application No. 6-122597discloses astructure in which an organic salt such as carbonate, nitrate, orsulfate is used as a target for pulse-laser deposition of an oxide thatis unstable in the air at room temperature. In accordance with thistechnique, an inorganic salt contained in the target is decomposed bylaser irradiation, and adheres onto the substrate as a crystalline oxidethin film. In this technique, however, the non-decomposed part of theinorganic salt may also adhere onto the substrate, or a nitride oxidegas or sulfur oxide gas may react to the silicon substrate. To obtain ahigh-quality crystalline thin film, it is preferable to perform filmformation in a vacuum space. However, an inorganic salt often containswater of crystallization in the crystals. When decomposed by laserirradiation, such crystallization water greatly reduces the vacuum. Theabove facts disturb the crystal structure of the oxide, and become ahindrance to formation of a high-quality crystalline thin film.

SUMMARY OF THE INVENTION

[0019] A general object of the present invention is to providesemiconductor devices, methods of forming an epitaxial film, and laserablation devices in which the above disadvantages are eliminated.

[0020] A more specific object of the present invention is to provide amethod of forming a single-crystal oxide thin film through epitaxialgrowth on a single-crystal silicon substrate, and a semiconductor devicethat includes such a single-crystal oxide thin film formed on asingle-crystal silicon substrate.

[0021] Another specific object of the present invention is to provide amethod of forming a single-crystal oxide epitaxial thin film having aperovskite structure with a high crystal orientation on a single-crystalsilicon substrate at such a vacuum that can be attained by an O-ringseal generally used in a simple vacuum device.

[0022] The above objects of the present invention are achieved by asemiconductor device that includes: a single-crystal silicon substrate;and a single-crystal oxide thin film having a perovskite structureformed through epitaxial growth on the single-crystal silicon substrate.In this semiconductor device, the single-crystal oxide thin film isdirectly in contact with a surface of the single-crystal siliconsubstrate, and contains a bivalent metal that is reactive to silicon.

[0023] The above objects of the present invention are also achieved by asemiconductor device that includes: a single-crystal silicon substrate;a single-crystal oxide thin film having a perovskite structure formedthrough epitaxial growth on the single-crystal silicon substrate; and anamorphous silicon layer interposed between the single-crystal siliconsubstrate and the single-crystal oxide thin film.

[0024] The above objects of the present invention are also achieved by asemiconductor device that includes: a single-crystal silicon substrate;a first single-crystal oxide thin film having a sodium chloridestructure formed through epitaxial growth on the single-crystal siliconsubstrate; and a second single-crystal oxide thin film having aperovskite structure formed through epitaxial growth on the firstsingle-crystal oxide thin film. This semiconductor device ischaracterized by the first single-crystal oxide thin film selected fromthe group consisting of CaO, SrO, and BaO.

[0025] The above objects of the present invention are also achieved by asemiconductor device that includes: a single-crystal silicon substrate;a first single-crystal oxide thin film having a sodium chloridestructure formed through epitaxial growth on the single-crystal siliconsubstrate; a second single-crystal oxide thin film having a perovskitestructure formed through epitaxial growth on the first single-crystaloxide thin film; and an amorphous layer interposed between thesingle-crystal silicon substrate and the first single-crystal oxide thinfilm.

[0026] The above objects of the present invention are also achieved by amethod of forming an epitaxial film, which method includes the steps of:forming a plume by irradiating a target containing a bivalent metalcarbonate with a laser beam; developing a bivalent metal oxide film fromthe bivalent metal carbonate through epitaxial growth on asingle-crystal silicon substrate set in a passage of the plume; andheating a surface of the target with an independent heat sourcedifferent from the laser beam, thereby producing a single-crystal oxideepitaxial film.

[0027] The above objects of the present invention are also achieved by alaser ablation device that includes: a processing chamber that isexhausted by an exhausting system; a processed substrate that is heldwithin the processing chamber; a target that is provided in theprocessing chamber and faces the processed substrate; an optical windowthat is provided in the processing chamber and corresponds to an opticalpath of the laser beam irradiating the target; and a heat source that isprovided in the processing chamber and covers a space between theprocessed substrate and the target.

[0028] In accordance with the present invention, when a perovskite oxidefilm is formed through epitaxial growth on a single-crystal siliconsubstrate by a laser ablation technique, with a single-crystal oxidefilm having a sodium chloride structure formed as an intermediate layer,carbonate is used as an ablation target for the oxide film as theintermediate layer. The surface of the target is heated with a heatsource other than a laser beam, or; more preferably, the plume itself isheated by the heat source, so that an oxide film containing a metalreactive to the silicon substrate can be formed as the intermediatelayer through epitaxial growth. A perovskite oxide film is then formedthrough epitaxial growth on such an intermediate layer, thereby forminga perovskite single-crystal oxide thin film containing a bivalent metalreactive to silicon on the silicon substrate.

[0029] The above and other objects and features of the present inventionwill become more apparent from the following description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 shows the crystal orientation of an epitaxial film obtainedin accordance with the present invention;

[0031]FIG. 2 shows the structure of a laser ablation device used inaccordance with the present invention;

[0032]FIGS. 3A through 3C illustrate a process for forming a SrTiO₃epitaxial film on a single-crystal silicon substrate in accordance witha first embodiment of the present invention;

[0033]FIG. 4A shows an X-ray diffraction pattern of a SrTiO₃ filmobtained when the plume is not heated in the device shown in FIG. 2;

[0034]FIG. 4B shows an X-ray diffraction pattern of a SrTiO₃ filmobtained when the plume is heated in the device shown in FIG. 2;

[0035]FIG. 5 shows the relationship between an oxygen partial pressureand the half value width of the x-ray rocking curve (FWHM) of a SrTiO₃film obtained with the device shown in FIG. 2;

[0036]FIGS. 6A through 6C illustrate a process for forming a (Ba,Sr)TiO₃ epitaxial film on a single-crystal silicon substrate inaccordance with a second embodiment of the present invention;,

[0037]FIG. 7 shows an X-ray diffraction pattern obtained with thestructure shown in FIG. 6C;

[0038]FIG. 8 shows an X-ray pole pattern obtained with the structureshown in FIG. 6C;

[0039]FIG. 9 shows X-ray diffraction patterns obtained with thestructure shown in FIG. 6C;

[0040]FIGS. 10A through 10D illustrate a process for forming a BaTiO₃epitaxial film on a single-crystal silicon substrate, with a SiOx filminterposed in between, in accordance with a third embodiment of thepresent invention;

[0041]FIGS. 11A through 11D illustrate a process for forming a SrTiO₃epitaxial film on a single-crystal silicon substrate, with a SiOx filmand a SrO epitaxial film interposed in between, in accordance with afourth embodiment of the present invention;

[0042]FIG. 12 shows an X-ray diffraction pattern obtained with thestructure shown FIG. 11D;

[0043]FIG. 13 shows an X-ray pole pattern obtained with the structureshown in FIG. 11A;

[0044]FIG. 14 shows X-ray diffraction patterns obtained with thestructure shown in FIG. 11D;

[0045]FIGS. 15A through 15C illustrate a process for forming a PZTepitaxial film on a single-crystal silicon substrate in accordance witha fifth embodiment of the present invention;

[0046]FIGS. 16A through 16C illustrate a process for forming a PLZT filmon a single-crystal silicon substrate in accordance with a sixthembodiment of the present invention; and

[0047]FIG. 17 shows the structure of a FeRAM in accordance with aseventh embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] The following is a description of embodiments of the presentinvention, with reference to the accompanying drawings.

[0049] The inventor(s) of the present invention made an intensive studyon the intermediate layer that is interposed between a (001)-orientatedsingle-crystal oxide film having a perovskite structure and the(001)-plane of a single-crystal silicon substrate on which thesingle-crystal oxide film is developed through epitaxial growth. As aresult, it was found that, when a SrO film, a CaO film, or a BaO filmhaving a sodium chloride structure is interposed as the intermediatelayer, the epitaxial growth of the intermediate layer film was seensubstantially in the corresponding directions of a-axis, b-axis andc-axis in a so-called cube-on-cube relationship with the siliconsubstrate, as shown in FIG. 1. It was also found that, when an oxidethin film having a perovskite structure formed through epitaxial growthon the above intermediate layer, the c-axis direction of the formedperovskite film corresponds to the c-axis direction of the intermediatelayer, whereas the a-axis and b-axis of the perovskite film arerotationally shifted from the a-axis and b-axis of the intermediatelayer by approximately 45 degrees. It is believed, that the rotationalshifting occurs to maintain the lattice constant conformity between theperovskite film and the intermediate layer having a sodium chloridestructure.

[0050] The above perovskite film maintains a (001)-orientation, and hasa polarization direction at right angle with respect to the surface ofthe silicon substrate. Accordingly, the piezoelectric effect and theferroelectric hysteresis become the maximum with such a (001)-orientatedfilm, which can be effectively employed in a FeRAM or a piezoelectricactuator.

[0051] An alkaline earth metal easily forms an oxide by taking oxygenfrom silicon For this reason, a sodium chloride structure oxidecontaining an alkaline earth metal formed as an intermediate layer incontact with a silicon substrate can restrict the formation of a SiOxamorphous layer on the interface between the intermediate layer and thesilicon substrate. Meanwhile, an alkaline earth metal oxide having asodium chloride structure is unstable and reactive to moisture or carbondioxide in the air. This makes an alkaline earth metal difficult tohandle, and causes a problem in presenting unstable film properties in asemiconductor device.

[0052] In the present invention, the intermediate layer and perovskitefilm is formed by a laser ablation technique, and an inorganic salt suchas carbonate is used, instead of an alkaline earth metal or oxide, as anablation target. If a pulse layer deposition process is performeddirectly on an inorganic salt target, however, the inorganic saltadheres to the substrate, and hinders the formation of a thin filmhaving a high crystal orientation. In order to solve this problem, thefollowing experiments were conducted in accordance with the presentinvention

[0053] 1) A carbonate target was temporarily heated at 700° C. or higherso as to decompose the carbonate on the surface of the carbonate targetinto oxide and carbon dioxide, which were subjected to laser ablation.

[0054] 2) The carbonate target was introduced into an oxygen atmospherechamber, and laser ablation was conducted at 700° C.

[0055] 3) The carbonate target was irradiated with a laser beam so as togenerate a plume, and the plume was heated to facilitate thedecomposition of the carbonate into oxides.

[0056] By conducting any of the above processes, the carbonated on thetarget can be effectively decomposed into oxide and carbon dioxide, andan alkaline earth metal oxide film having an excellent crystallizationcan be formed through epitaxial growth on the silicon substrate. Inplace of carbonate, nitrate or sulfate can be employed as a target, butalkaline earth metal nitrate has a melting point of approximately 500°C. and starts decomposing into oxide at this temperature. Since thismelting point is lower than the thin-film deposition temperature(between 600° C. and 800° C.), it is necessary with a nitrate target tochange the deposition substrate temperature and the target temperaturein a laser ablation device, and the structure of the deposition devicebecomes more complicated. As for sulfate, the decomposition temperatureis approximately 1100° C., which is higher than the depositiontemperature. If the temperature of the sulfate target is lowered to thedeposition substrate temperature, the decomposition rate becomes lowerand therefore disadvantageous. For these reasons, carbonate isconsidered to be the most preferable as a target.

[0057]FIG. 2 shows the structure of a laser ablation device 1 used inaccordance with the present invention. As shown in FIG. 2, the laserablation device 1 includes a processing chamber 10 that is exhausted bya pump 16, and, in the processing chamber 10, a single-crystal siliconsubstrate is held as a processed substrate 13 on a heater 12A set on arotational axis 13A.

[0058] The processing chamber 10 also accommodates a target 15 facingthe processed substrate 13. Upon the target 15, a high-powered laserbeam 11 is focused through a window 10A. As the laser beam 11, KrF orArF excimer laser, femtosecond laser, or Nd:YAG higher harmonic lasercan be used.

[0059] With the irradiation with the laser beam 11, the surface of thetarget 15 is instantly atomized, and a flame 14 called “plume” is formedas a result. The processed substrate 13 is set in a passage of the plume14, and the oxide that has been atomized on the surface of the target 15and transported in the form of a plume is deposited through epitaxialgrowth on the processed substrate 13.

[0060] The target 15 is preferably made of a sintered body of carbonate,and spontaneously rotates during the irradiation with the laser beam 11so that the entire surface can be irradiated with the laser beam 11. Thetarget 15 is held on a rotational arm 17A supported by the rotationalaxis 17, but the rotation of the rotational, axis 17 can move a nexttarget 15A to the irradiation point of the laser beam 11.

[0061] In the laser ablation device 10 shown in FIG. 2, a space 12Cbetween the target 15 and the processed substrate 13 is covered with aheater 12B, which controls the temperature of the space 12C atapproximately 800° C. By controlling the temperature of the space 12C,the heater 12B heats the target 15, and an alkaline earth metal oxide isformed on the surface of the target 15. The oxide formed in this manneris more stable than carbonate, and can be transported inside the plume14 and deposited on the processed substrate 13. Also, with the heater12B heating the plume 14 itself, the chemical stability of the oxideclusters being transported in the plume 14 can be increased.

[0062] The oxide that has reached the processed substrate 13 in theabove manner does not easily react to silicon that constitutes theprocessed substrate 13. The formation of the oxide requires an oxygenpartial pressure of only 10⁻¹ Torr or so. On the contrary, theconventional formation of oxide by a chemical vapor deposition (CVD)technique requires an oxygen partial pressure of several 2 to 10 Torr todecompose an organic metal raw material. In the laser ablation deviceshown in FIG. 2, therefore, an amorphous SiOx layer can be preventedfrom forming on the surface of the silicon substrate. 13, and ahigh-quality alkaline earth metal oxide thin film can be formed throughepitaxial growth directly on the surface of the single-crystal siliconsubstrate 13.

[0063] In the laser ablation device shown in FIG. 2, it is also possibleto replace the target 15 with the next target 15A made of perovskitestructure oxide ceramics, and to form a perovskite structure oxide filmon the epitaxial thin film made of alkaline earth metal oxide. Since thesurface of the processed substrate 13 is covered with the epitaxial filmhaving a sodium chloride structure on which a perovskite structure oxidecan easily grows, a perovskite structure oxide film can be easily formedthrough epitaxial growth.

[0064] Although only the method of heating the targets 15 and 15A duringthe film formation is shown in the schematic view of FIG. 2, the presentinvention is not limited to this example. For instance, it is alsopossible to heat a target in an electric furnace or another processingchamber. In such a method, an oxide formed on a part of the surface of acarbonate target is transported to the processing chamber 10, and thefilm formation is then conducted. The method of heating both the target15 and the plume 14 as shown in FIG. 2 is the most effective, but amethod of heating either the target 15 or the plume 14 is also effectiveenough to improve the thin film crystallization.

[0065] [First Embodiment]

[0066]FIGS. 3A through 3C shows an epitaxial growth process of a singlecrystal SrTiO₃ film on a single-crystal silicon substrate using thelaser ablation device in accordance with a first embodiment of thepresent invention.

[0067] As shown in FIG. 3A, by a pulse laser deposition technique usinga SrCO₃ target as the target 15 shown in FIG. 2, a SrO film 52 having athickness of 5 to 6 nm is deposited on a silicon substrate 51 from whicha natural oxide film has been removed through a high-frequency, (HR)process. In the procedure shown in FIG. 3B, by a pulse laser depositiontechnique using a SrTiO₃ target as the target 15, a SrTiO₃ film 53having a thickness of 100 nm is deposited on the SrO film 52. In theprocedures shown in FIGS. 3A and 3B, the substrate temperature is set at800° C. by driving the heater 12A. In the procedures shown in FIG. 3A,laser ablation for the SrCO₃ target 15 is performed under a pressure of1×10⁻⁵ Torr for 0.25 minute, and then in an oxygen atmosphere of 1×10⁻⁴Torr for 0.25 minute. Meanwhile, in the procedure shown in FIG. 3B, thelaser ablation is performed in an oxygen atmosphere of 5×10⁻³ Torr for12 minutes.

[0068] In the procedure shown in FIG. 3B, Sr diffuses from the SrO film52 toward the SrTiO₃ film 53 formed thereon, as indicated by the arrowin FIG. 3B. As a result, the SrO film 52 substantially disappears, and asingle-crystal SrTiO₃ film 54 that is uniform in appearance is formed onthe silicon substrate 51, with an epitaxial relation ship maintainedbetween the single crystal SrTiO₃ film 54 and the silicon substrate 51,as shown in FIG. 3C.

[0069]FIG. 4A shows an X-ray diffraction pattern obtained in a casewhere only the heater 12A shown in FIG. 2 is driven while the heater 12Balso shown in FIG. 2 is not driven in the above described process. FIG.4B shows an X-ray diffraction pattern in a case where the heater 12Ashown in FIG. 2 is driven to maintain the space at approximately 800° C.

[0070] As can be seen from FIG. 4A, when only the substrate 13 is heatedto obtain a deposition film, the diffraction peak of the obtained SrTiO₃film is low, and the half value width (FWHM) is as great as 7.0. Thisshows the film quality of the obtained SrTiO₃ film is low. As shown inFIG. 4B, when the heater 12B as well as the heater 12A is driven to heatthe plume 14 and the target 15 at the same temperature of approximately800° C. as the substrate 13, the diffraction peak of the obtained SrTiO₃film is very high, reaching a value 10 times as great as the diffractionpeak shown in FIG. 4A, and the half value width (FWHM) decreases to 1.9.

[0071]FIG. 5 shows the relationship between the half value width (FWHM)and the deposition processing pressure of the obtained SrTiO₃ epitaxialfilm.

[0072] When conducting laser ablation on a alkaline earth metal oxide asa target, it is generally believed that a high vacuum must be maintainedso as to avoid deterioration of the target that is chemically unstablein the air. In the present invention, on the other hand, a stable targetmade of carbonate is used, and a high-quality perovskite thin film canbe formed in a low vacuum.

[0073] Referring to FIG. 5, when the pressure in the processing chamber10 is 10⁻¹ Torr or higher, or 10⁻⁴ Torr or lower, the X-ray peak halfvalue width (FWHM) of the SrTiO₃ film increases. When the pressure inthe processing chamber 10 is in a range of 10⁻¹ Torr to 10⁻⁴ Torr, thehalf value width becomes the smallest. This implies that an oxygendeficiency in the deposited SrTiO₃ film 54 is effectively compensatedunder an oxygen partial pressure of this range.

[0074] In the above manner, an exceptionally high-quality single-crystalSrTiO₃ film can be formed through epitaxial growth by heating the plume14 and the SrO target 15 using carbonate during laser irradiation in thelaser ablation device of FIG. 2 in accordance with the first embodimentof the present invention.

[0075] [Second Embodiment]

[0076]FIGS. 6A through 6C illustrate the procedures for forming a (Ba,Sr)TiO₃ film on a single-crystal silicon substrate in accordance with asecond embodiment of the present invention. In the drawings, the samecomponents as in the first embodiment are denoted by the same referencenumerals as well, and explanations for those components are omitted fromthe following description.

[0077] Referring to FIG. 6A, a single-crystal SrO film 52 is formed onthe single-crystal silicon substrate 51 by driving the heater 12A andthe heater 12B of the laser ablation device of FIG. 2 so as to heat theplume 14 and the target 15, as well as the processed substrate 13, at800° C. In the procedure shown in FIG. 6A, the deposition of thesingle-crystal SrO film 52 is conducted under a pressure of 1×10⁻⁵ Torrfor 8 minutes, and then under a pressure of 5×10⁻⁴ Torr at 800° C. for0.5 minute.

[0078] In this embodiment, the thickness of the single-crystal SrO filmis several nanometers or less. In the procedure shown in FIG. 6B, asingle-crystal BaTiO₃ film 63 having a thickness of approximately 200 nmis formed on the single-crystal SrO film 52 by a pulse laser depositiontechnique using BaTiO₃ as a target. Laser ablation is then conducted inan oxygen atmosphere of 1×10⁻² Torr for 10 minutes.

[0079] In the procedure shown in FIG. 6B, the Sr in the SrO film 52diffuses into the BaTiO₃ film 63, which then changes into asingle-crystal (Ba, Sr)TiO₃ film 64, as shown in FIG. 6C. As s result,the (Ba, Sr) TiO₃ film 64 that is directly in contact with the siliconsubstrate 51 can be obtained, with an epitaxial relationship beingmaintained between the silicon substrate 51 and the (Ba, Sr)TiO₃ film64.

[0080]FIG. 7 shows an X-ray diffraction pattern of the (Ba, Sr)TiO₃ film64 obtained in the above manner.

[0081] As shown in FIG. 7, in the above structure, only the reflectionsfrom the (001)-plane and the (002)-plane of the (Ba, Sr)TiO₃ areobserved, while the reflection from the SrO film is not. This impliesthat the (Ba, Sr)TiO₃ film 64 is a single-crystal film having a(001)-orientation.

[0082]FIG. 8 shows the results of X-ray pole measurement conducted onthe structure shown in FIG. 6C.

[0083] The X-ray pole pattern is a diffraction pattern obtained bytilting and then rotating a sample in a normal line direction withrespect to a certain diffraction peak. If the sample is particles or apolycrystalline body, a diffraction peak can be observed, regardless ofthe tilt angle or the rotation angle. If the sample is a single-crystalfilm, on the other hand a diffraction peak is observed only with respectto a certain tilt angle and a certain rotation angle.

[0084] The X-ray pole pattern is obtained with respect to the(011)-diffraction peak of the (Ba, Sr)TiO₃film 64. In this pattern, fourpoles corresponding to the (011)-plane are shown, which clearlyindicates that the obtained (Ba, Sr)TiO₃ film 64 is a single-crystalfilm and has 4 rotation symmetry axes. It can also be seen from FIG. 8that the (Ba, Sr)TiO₃film 64 does not have twins formed therein.

[0085]FIG. 9 shows the comparison results of rotation angles φ in theX-ray pole pattern measurement of FIG. 8 conducted on the siliconsubstrate 51 and the (011)-plane of the (Ba, Sr)TiO₃ film 64.

[0086] As shown in FIG. 9, the (011)-plane of the (Ba, Sr)TiO₃ film 64is shifted from the (011)-plane of the silicon substrate 51 byapproximately 45 degrees, and the relationship described with referenceto FIG. 1 is established between the silicon substrate 51 and the (Ba,Sr)TiO₃ film 64.

[0087] [Third Embodiment]

[0088]FIGS. 10A through 10D shows an epitaxial growth process of asingle-crystal BaTiO₃ film on a single-crystal silicon substrate inaccordance with a third embodiment of the present invention. In thedrawings, the same components as in the foregoing embodiments aredenoted by the same reference numerals as well, and explanations forthose components are omitted from the following description.

[0089] Referring to FIG. 10A, a BaO film 62 having a thickness of 5 to 6mm is deposed on a silicon substrate film 51 from which a natural oxidefilm has been removed through a HF process, by a pulse laser depositiontechnique using a BaCO₃ target as the target 15 shown in FIG. 2. In aprocedure shown in FIG. 10B, a BaTiO₃ film 63 having a thickness of 200mm is deposited on the BaO film 62 by a pulse laser deposition techniqueusing a BaTiO₃ target as the target 15. During the procedures shown inFIGS. 10A and 10B, the heaters 12A and 12B are driven so as to maintainthe substrate 13, the target 15, and the plume 14 at 800° C. In theprocedure shown in FIG. 10A, the BaCO₃ target is subjected to laserablation under a pressure of 5×10⁻⁶ Torr for 1 minute and then in anoxygen atmosphere of 1×10⁻² Torr for 2 minutes. In the procedure shownin FIG. 10B, the laser ablation is performed in an oxygen atmosphere of1×10⁻² Torr for 10 minutes.

[0090] In this manner, the Ba in the BaO film 62 diffuses toward theBaTiO₃ film 63 deposited thereon, as indicated by the arrow in FIG. 10B.As a result, the BaO film 62 substantially disappears, and asingle-crystal BaTiO₃film 64 that is uniform in appearance is formed onthe silicon substrate 51, as shown in FIG. 10C, with an epitaxialrelationship being maintained between the silicon substrate 51 35 andthe single-crystal BaTiO₃ film 64.

[0091] In this embodiment, the structure shown in FIG. 10C is furthersubjected to heat treatment in an oxygen atmosphere at 1100° C. for 5hours. As a result, an amorphous phase SiOx layer 51A having a thicknessof approximately 50 nm is formed between the BaTiO₃ film 63 and thesilicon substrate 51, as shown in FIG. 10D.

[0092] As described above, in accordance with this embodiment, a BaTiO₃film can be formed through epitaxial growth on a single-crystal siliconsubstrate, with an amorphous-phase SiOx layer being interposed inbetween.

[0093] [Fourth Embodiment]

[0094]FIGS. 11A through 11D illustrate an epitaxial growth process of asingle-crystal SrTiO₃ film on a single-crystal silicon substrate inaccordance with a fourth embodiment of the present invention. In thedrawings, the same components as in the foregoing embodiments aredenoted by the same reference numerals, and explanations for thosecomponents are omitted from the following description.

[0095] Referring to FIG. 11A, a SrO film 52 is deposited on a siliconsubstrate 51 from which a natural oxide film is removed through a HFprocess, by a pulse laser deposition technique using a SrCO₃ target asthe target 15 shown in FIG. 2. More specifically, the substrate 13, thetarget 15, and the plume 14 of the laser ablation device 1 aremaintained at 800° C., and the SrCO₃ target is subjected to laserablation under a pressure of 1×10⁻⁵ Torr for 1 minute and then in anoxygen atmosphere of 5×10⁻⁴ Torr for 8 minutes, thereby producing theSrO film 52. In a procedure shown in FIG. 11B, by a pulse laserdeposition technique using a SrTiO₃ target as the target 15, laserablation is performed in an oxygen atmosphere of 5 ×10⁻⁴ Torr for 10minutes, so as to deposit a SrTiO₃ film 53 on the SrO film 52. Duringthe procedures shown in FIGS. 11A and 11B, the heaters 12A and 12B aredriven so as to maintain the substrate 13, the target 15, and the plume14 at 800° C.

[0096] In this embodiment, the Sr in the SrO film 52 diffuses toward theSrTiO₃film 53, as indicated by the arrow in FIG. 11B. However, the SrOfilm 52 does not completely disappear, but remain as an epitaxialsingle-crystal film between a SrTiO₃ film 54 and the silicon substrate51, as shown in FIG. 11C.

[0097]FIG. 12 shows an X-ray diffraction pattern measured with respectto the structure shown in FIG. 11C.

[0098] In FIG. 12, the reflection from the (002)-plane of the SrO isobserved as well as the reflections from the (001)-plane and (002)-planeof the SrTiO₃. This indicates that the structure shown in FIG. 11C has alamination structure of a SrO film and a SrTiO₃ film.

[0099]FIG. 13 shows the results of X-ray pole measurement conducted onthe structure shown in FIG. 11C.

[0100] The X-ray pole pattern shown in FIG. 13 is obtained throughmeasurement of the (011)-diffraction peak of the SrO film 52, and showsfour poles corresponding to the (011)-plane. This clearly indicates thatthe obtained SrO film 52 is a single-crystal film and has four rotationsymmetry axes. It can also be seen from FIG. 13 that the SrO film 52does not has twin crystals formed therein.

[0101]FIG. 14 shows comparison results of rotational angles φ throughthe X-ray pole pattern measurement conducted on the silicon substrate51, the SrO film 52, and the (011)-plane of the SrTiO₃ film 54.

[0102] As can be seen from FIG. 14, the (022)-plane of the siliconsubstrate 51 completely corresponds to the (011)-plane of the SrO film52, and the cube-on-cube relationship shown in FIG. 1 is established.Meanwhile, the (011)-plane of the SrO film 52 and the (011)-plane of theSrTiO₃ film are shifted from each other by approximately 45 degrees.

[0103] In this embodiment, the structure shown in FIG. 11C is furthersubjected to heat treatment in an oxygen atmosphere at 1100° C. for 5hours. As a result, an amorphous-phase SiOx layer 51A having a thicknessof approximately 50 nm is formed between the SrO film 52 and the siliconsubstrate 51, as shown in FIG. 11D.

[0104] In the above manner, in accordance with this embodiment, a SrTiO₃film can be formed through epitaxial growth on a single-crystal siliconsubstrate, with an amorphous-phase SiOx layer being interposed inbetween.

[0105] [Fifth Embodiment]

[0106]FIGS. 15A through 15C illustrate an epitaxial growth process of asingle-crystal PZT (Pb (Zr, Ti) O₃) film on a single-crystal siliconsubstrate in accordance with a fifth embodiment of the presentinvention. In the drawings, the same components as in the foregoingembodiments are denoted by the same reference numerals as well, andexplanations for those components are omitted from the followingdescription.

[0107] Referring to FIG. 15A, a SrO film 52 is formed on a siliconsubstrate 51 by a pulse laser deposition technique using a SrCO₃ target.More specifically, the substrate temperature, the target temperature,and the plume temperature are set at 800, and laser ablation inaccordance with the pulse laser deposition technique is performed undera pressure of 1×10⁻⁶ Torr for 1 minute and then in an oxygen atmosphereof 5×10⁻⁴ Torr for 1 minute, thereby producing the SrO film 52.

[0108] In the next procedure shown in FIG. 15B, the target is changed toPZT, and the laser ablation in accordance with the pulse laserdeposition technique is performed in an oxygen atmosphere of 1×10⁻¹Torr, so as to form a PZT film 73 on the SrO film 52.

[0109] At the time of deposition of the PZT film 73, the Sr in the SrOfilm 52 diffuses toward the PZT film 73, as indicated by the arrow inFIG. 15B. As a result, a PZT film 74 is formed and located directly incontact with the silicon substrate 51, as shown in FIG. 15C, with anepitaxial relationship being maintained in between.

[0110] [Sixth Embodiment]

[0111]FIGS. 16A through 16C illustrate an epitaxial growth process of asingle-crystal PLZT((Pb, La) (Zr, Ti) O₃) film on a single-crystalsilicon substrate in accordance with a sixth embodiment of the presentinvention. In the drawings, the same components as in the foregoingembodiments are indicated by the same reference numerals as well, andexplanations for those components are omitted from the followingdescription.

[0112] Referring to FIG. 16A, a BaO film 62 is formed on a siliconsubstrate 51 by a pulse laser deposition technique using a BaCO₃ target.More specifically, the substrate temperature, the target temperature,the plume temperature are set at 800° C., and laser ablation inaccordance with the pulse laser deposition technique is performed undera pressure of 5×10⁻⁶ Torr for 2 minutes and then in an oxygen atmosphereof 1×10⁻² Torr for 8 minutes, thereby producing the BaO film 62.

[0113] In the next procedure shown in FIG. 16B, the target is changed toPLZT, and the laser ablation according to the pulse laser depositiontechnique is performed by an oxygen atmosphere of 1×10⁻¹ Torr, so as toform a PLZT film 83 on the BaO film 62.

[0114] At the time of deposition of the PLZT film 83, the Ba in the BaOfilm 62 diffuses toward the PLZT film 83, as indicated by the arrow inFIG. 16B. As a result, a PLZT film 84 is formed and located directly incontact with the silicon substrate 51, as shown in FIG. 16C, with anepitaxial relationship being maintained in between.

[0115] In the above embodiments, the single-crystal oxide films havingsodium chloride structures are not limited to a SrO film and a BaO film,but an MgO film or a Cao film can also be employed. When asingle-crystal MgO film is to be formed, a MgCO₃ target should be usedin the laser ablation device of FIG. 2. When a single-crystal CaO filmis to be formed, a CaCO₃ target should be used.

[0116] Also in the above embodiments, the perovskite single-crystaloxide thin films are not limited to a SrTiO₃ film, a BaSrTiO₃ film, a(Ba, Sr) TiO₃ film, a SrRuO₃ film, a PZT film, and a PLZT film, but afilm containing Mg or Ca as a bivalent metal can also be employedAlternatively, the perovskite single crystal oxide films may contain Ag,Al, Ba, Bi, Ca, Ce, Cd, Co, Cu, Dy, Eu, Fe, Ga, Gd, Hf, I, In, La, Li,Mn, Mo, Na, Nb, Ni, Os, Pa, Pb, Pr, Re, Rh, Sb, Sc, Sm, Sn, Sr, Ta, Te,Th, Tl, U, V, W, Y, Sm, Yb, or Zr.

[0117] [Seventh Embodiment]

[0118]FIG. 17 shows the structure of a ferroelectric memory (FeRAM) 100in accordance with a seventh embodiment of the present invention.

[0119] As shown in FIG. 17, the FeRAM 100 is formed on a single-crystalsubstrate 101, and contains a single-crystal PZT film 103 formed throughepitaxial growth on the single-crystal silicon substrate 101, with aSiOx thin film 102 being interposed in between, and a Pt gate electrode104 formed on the single-crystal PZT film 103. In the single-crystalsilicon substrate 101, an N-type or p-type diffusion regions 101A and101B are formed in such positions as to flank the area corresponding tothe Pt gate electrode 104.

[0120] In this FeRAM 100, a write voltage is applied to the gateelectrode 104, so that polarization is induced in the single-crystal PZTfilm 103 to change the threshold voltage of the transistor.

[0121] At a time of reading, a read voltage is applied to the gateelectrode 104, and the conductance between the diffusion regions 101Aand 101B is detected By doing so, information written in the form ofremanence in the PZT film 103 can be read out.

[0122] In accordance with this embodiment, the single-crystal PZT filmhas such an orientation that the c-axis direction is perpendicular tothe principal plane while the polarization occurs in the c-axisdirection. With this structure, the maximum value of the remanence isobtained. Accordingly, the write voltage can be minimized in this FeRAM.

[0123] It should be noted that the present invention is not limited tothe embodiments specifically disclosed above, but other variations andmodifications may be made without departing from the scope of thepresent invention.

What is claimed is:
 1. A semiconductor device comprising: asingle-crystal silicon substrate; and a single-crystal oxide thin filmhaving a perovskite structure formed through epitaxial growth on thesingle-crystal silicon substrate, said single-crystal oxide thin filmbeing directly in contact with a surface of the single-crystal siliconsubstrate, and containing a bivalent metal that is reactive to silicon.2. The semiconductor device as claimed in claim 1, wherein the bivalentmetal is any bivalent metal but Sr.
 3. The semiconductor device asclaimed in claim 1, wherein the single-crystal oxide thin film isselected from the group consisting of PbTiO₃, PbZrO₃, Pb(Zr, Ti)O₃, (Pb,La)(Zr, Ti)O₃, BaTiO₃, and (Ba, Sr)TiO₃.
 4. A semiconductor devicecomprising: a single-crystal silicon substrate; a single-crystal oxidethin film having a perovskite structure formed through epitaxial growthon the single-crystal silicon substrate; and an amorphous silicon layerinterposed between the single-crystal silicon substrate and thesingle-crystal oxide thin film.
 5. A semiconductor device comprising: asingle-crystal silicon substrate; a first single-crystal oxide thin filmhaving a sodium chloride structure formed through epitaxial growth onthe single-crystal silicon substrate; and a second single-crystal oxidethin film having a perovskite structure formed through epitaxial growthon the first single-crystal oxide thin film, said first single-crystaloxide thin film being selected from the group consisting of CaO, SrO,and BaO.
 6. The semiconductor device as claimed in claim 4, wherein thesingle-crystal oxide thin film contains a bivalent metal selected fromthe group consisting of Sr, Ba, Pb, and La.
 7. The semiconductor deviceas claimed in claim 4, wherein the single-crystal oxide thin film isselected from the group consisting of PbTiO₃, PbZrO₃, Pb(Zr, Ti)O₃, (Pb,La) (Zr, Ti)O₃, BaTiO₃, (Ba, Sr)TiO₃, and SrTiO₃.
 8. A semiconductordevice comprising: a single-crystal silicon substrate; a firstsingle-crystal oxide thin film having a sodium chloride structure formedthrough epitaxial growth on the single-crystal silicon substrate; asecond single-crystal oxide thin film having a perovskite structureformed through epitaxial growth on the first single-crystal oxide thinfilm; and an amorphous layer formed between the single-crystal siliconsubstrate and the first single-crystal oxide thin film.
 9. Thesemiconductor device as claimed in claim 8, wherein the firstsingle-crystal oxide thin film is selected from the group consisting ofMgO, CaO, SrO, and BaO.
 10. The semiconductor device as claimed in claim8, wherein the second single-crystal oxide thin film is selected fromthe group consisting of PbTiO₃, PbZrO₃, Pb(Zr, Ti)O₃, (Pb, La) (Zr,Ti)O₃, BaTiO₃, (Ba, Sr)TiO₃, and SrTiO₃.
 11. A method of forming anepitaxial film, comprising the steps of: forming a plume by irradiatinga target containing a bivalent metal carbonate with a laser beam;developing a bivalent metal oxide film from the bivalent metal carbonatethrough epitaxial growth on a single-crystal silicon substrate set in apassage of the plume; and heating a surface of the target with anindependent heat source different from the laser beam, thereby producinga single-crystal oxide. epitaxial film.
 12. The method as claimed inclaim 11, wherein the step of heating the surface of the target isperformed at the same time as the irradiation with the laser beam. 13.The method as claimed in claim 11, further comprising the step ofheating the plume.
 14. The method as claimed in claim 11, wherein thestep of heating the surface of the target is performed prior to theirradiation with the laser beam.
 15. The method as claimed in claim 11,wherein the step of heating the surface of the target is performed atsuch a temperature that the carbonate decomposes on the surface of thetarget.
 16. The method as claimed in claim 11, further comprising thestep of forming an oxide film having a perovskite structure throughepitaxial growth on the single-crystal oxide epitaxial film byirradiating another target with a laser beam.
 17. A laser ablationdevice comprising: a processing chamber that is exhausted by anexhausting system; a processed substrate that is held within theprocessing chamber; a target that is provided in the processing chamberand faces the processed substrate; an optical window that is provided inthe processing chamber and corresponds to an optical path of the laserbeam irradiating the target; and a heat source that is provided in theprocessing chamber and covers a space between the processed substrateand the target.